Global-scale changes in the area of atoll islands during the 21st century
Introduction
Many of the world’s coasts are considered threatened by erosion as the result of a multitude of anthropogenic and natural stressors (Zhang et al., 2004; Mentaschi et al., 2018). These stressors, generated at both local and global scales, are expected to accelerate as sea level rise driven by climate change places pressure on coastal systems and the ecosystems, communities and economies in which they support. Coastal areas, particularly those along heavily urbanised coasts, are engineered at an accelerating rate, pushing coastlines seaward as wetlands are converted to land, shorelines are advanced and armoured and in places entirely new islands are constructed (Bisaro, 2019). At the forefront of discussions surrounding the erosional impacts of sea level rise on coastal systems are low-lying atoll islands. Atoll islands are typically low-lying landforms, largely found within the world’s tropical oceans. For the nations of Tuvalu, the Maldives and the Republic of the Marshall Islands are the only sites of habitable land. The islands are a product of the deposition of reef-derived material and are typically only a few meters in elevation above sea level. As a result of their small size and low elevation, atoll islands are often considered among the most vulnerable systems with respect to the impacts of climate change (McLean and Kench, 2015).
The future persistence of atoll islands both as geologic features and as sites of human habitation is uncertain. Anthropogenic climate change is expected to increase sea level and ocean temperature and alter the frequency and intensity of tropical storms (Bacmeister et al., 2018). Similarly, changes in ocean temperature and chemistry are expected to reduce the protection services provided by surrounding coral reefs (Harvey et al., 2018) and reduce the generation of biogenic sediment (Perry et al., 2011), leaving islands vulnerable to shoreline erosion, threatening coastal infrastructure and the stability of the islands themselves (Kench and Mann, 2017). Sea level rise, in concert with changes in ocean swell, is expected to increase the frequency of overtopping and inundation of islands, resulting in damage to infrastructure and the intrusion of saltwater into groundwater reserves (Storlazzi et al., 2018). The exposure to these changing environmental conditions has led to the grim prognosis that many atoll-dwellers will be climate change refugees in the coming decades, as their islands become uninhabitable or even disappear (Betzold, 2015).
Many atoll islands have been subjected to the same engineering interventions observed along continental coasts, including land reclamation, shoreline armouring and island building. The human modification of island shorelines is widespread within populated atoll islands, as nations seek to protect urban infrastructure and create more land to accommodate internal migration and economic development (Duvat, 2020; Duvat and Magnan, 2019; Fallati et al., 2017). The active manipulation of island shorelines has been proposed, and in several cases implemented, as an adaptation option for atoll nations in the face of continued sea level rise (Esteban et al., 2019). The scale and drivers of such interventions varies, from local scale reclamations for the development of individual dwellings, to the construction of entirely new land masses at a scale to support populations of several thousand people (Biribo and Woodroffe, 2013; Duvat and Magnan, 2019). Several studies have documented the construction of islands within the South China Sea, where several nations have built islands as tensions over marine boundary disputes have increased (Barnes and Hu, 2016; Asner et al., 2017). Similarly, noteworthy island building has occurred within the Maldives, where islands have been expanded to house tourist resorts, as well as increase the availability of land for the local population and infrastructure (Speelman et al., 2017). Such large-scale reclamation projects have created new islands, such as Hulhumalé Island in the Maldives, which is 1.80-1.90 km2 and currently expanding further (Naylor, 2015; Stojanov et al., 2017). In contrast, most land reclamation and shoreline armouring on Pacific atolls has been smaller in scale to the island building observed in the South China Sea and Maldives. The reclamation has generally been restricted to piecemeal hardening of the shoreline of the densely populated atolls, although in rare cases, the construction of airports (Ford, 2011) and military bases (Hamylton and East, 2012) has resulted in significant island-building.
The prospect of coastal erosion is a key concern for atoll nations and features prominently in political and popular media discussions of the impacts of sea level rise on atoll islands (Donner, 2012). Such proclamations underpin the ‘disappearing islands’ narrative when discussing atoll futures. Despite the widely circulated assertion that atoll islands are actively eroding, however, little scientific evidence exists to support such blanket claims. Records do not exist of widespread, chronic erosion of atoll islands under current rates of sea level rise (Duvat, 2019). Rather, studies have shown that islands have persisted, and in many cases increased in size, since the mid-20th century (Kench et al., 2018; Duvat, 2019). These studies have generally revealed considerable intra- and inter-atoll variability with respect to observed changes in planform island configuration, with no discernible pattern of island change described to date, although in some studies, smaller islands have shown greater variability in change spatially and temporally (Kench et al., 2018; Aslam and Kench, 2017). Most studies have speculated on the role that reef-generated sediment delivery to the island shorelines must play in driving changes on the islands. Robustly identifying and attributing the specific processes responsible for island growth, however, remains a challenge (Duvat, 2019).
Over the past decade, multiple studies have adopted remote sensing approaches to investigate changes in the position of shorelines and the planform area of atoll islands (Table 1). These studies are underpinned by the comparison of planform island morphology interpreted from historic aerial photographs and recent satellite imagery. Studies rely on limited collections of mapping-quality aerial photographs that provide fragmentary records of island morphology from the mid-20th century. These photographs are compared with island morphology interpreted from modern, high-resolution (i.e. < 1 m) satellite imagery. The methodologies applied to analyse atoll island change using remote imagery rely on the interpretation and digitisation of islands within a Geographic Information System (GIS). This approach, while robust, relies on the manual interpretation of shorelines and is therefore subjective, time-consuming and costly. The constraints imposed by the difficulty and cost of acquiring historic and recent remote imagery, along with the interpretation costs, have contributed to a collection of studies with limited spatio-temporal coverage. These limitations further confound attempts to quantify global-scale island change, and to detect and attribute the drivers of change.
Despite concerns surrounding the vulnerability of atoll-islands and numerous local and national-scale assessment of shoreline change, few assessments have adequately assessed how the land area of atolls has changed in the 21st century at a global scale.
In this paper we address the following four questions: (1) To what extent has the land area changed on atolls over the 21 st century? (2) Is there regional variability in the changes in land area on atolls? (3) To what extent are anthropogenic influences key drivers of land area change on atoll islands (4) How can freely available, moderate-resolution satellite imagery contribute to global-scale monitoring of changes on low-lying atoll islands?
Section snippets
Field setting
Global estimates indicate at least 439 atolls, found largely in the tropical Pacific and Indian Oceans (Goldberg 2016). Three countries comprise entirely low-lying atolls: The Maldives, Marshall Islands and Tuvalu. Except for one raised coral island (Banaba), Kiribati comprises exclusively atolls, whereas the Federated States of Micronesia (FSM), Cook Islands and French Polynesia are home to several atolls. This study examined changes on land areas on 279 atolls throughout the Pacific and
Data sources
Several studies have utilised high resolution (i.e. sub-metre pixel size) optical satellite imagery to study land area change on atolls (Table 1). These images are typically commercial, however, which are costly to acquire and analyse at a large scale. In contrast, Landsat provides freely accessible imagery at a coarser resolution (30 m) to that typically employed in such studies. Landsat imagery is widely used for a range of analyses of land cover change and has recently provided global-scale
Global-scale change in the area of atoll islands
Between the oldest (1999–2001 or 1999–2002) and most recent (2017) composite images, the land area on the 221 atolls examined increased by 61.74 km2 from 1007.60 km2 to 1069.35 km2, a 6.1 % increase. Most of this increase, 38.89 km2, occurred between 2013 and 2017 (Fig. 3). The global-scale change in atoll island landmass was largely a product of an increase of island area in the Maldives and South China Sea (SCS), which account for 54.05 km2 (87.56 %) of the global increase in land area.
Global scale island change
This study has examined, for the first time, land area change on atolls at a global-scale. The study measures land area change in the major atoll nations including the Maldives, Marshall Islands, Kiribati and Tuvalu, as well as large numbers of atolls in Micronesia and French Polynesia. This examination of island change covers a spatial extent far greater than previous efforts to monitor land area change on atolls, which have not exceeded ten atolls in any given study (Table 1). Likewise, by
Conclusions
In summary, analysis of change in the land area of islands perched on 221 atolls, using satellite imagery, produced answers to the research questions posed in this paper. First, the study shows a large increase the area of atoll islands, with 61.74 km2 of land added between 2000 and 2017. s, results show that most changes in atoll land area have taken place in the Maldivian archipelago and on atolls in the South China Sea. Of the 61.74 km2 of land area increase observed in the study, 54.07 km2
Data availability
All Landsat imagery used in the study is publicly available. All code used in the analysis is available at https://github.com/geotrip/AtollIslandsInGEE
Declaration of Competing Interest
This research received no funding beyond a student scholarship to Andrew Holdaway. The authors are aware of no conflicts of interest with this research.
Acknowledgements
This work was supported by a University of Auckland Masters scholarship.
References (57)
- et al.
Reef island dynamics and mechanisms of change in Huvadhoo Atoll, Republic of Maldives, Indian Ocean
Anthropocene.
(2017) - et al.
Land reclamation and artificial islands: walking the tightrope between development and conservation
Glob. Ecol. Conserv.
(2017) Human-driven atoll island expansion in the Maldives
Anthropocene
(2020)- et al.
Drivers of shoreline change in atoll reef islands of the Tuamotu Archipelago, French Polynesia
Glob. Planet. Change
(2017) - et al.
Impacts of category 5 tropical cyclone fantala (April 2016) on Farquhar Atoll, Seychelles Islands, indian ocean
Geomorphology.
(2017) - et al.
Adaptation to sea level rise on low coral islands: lessons from recent events
Ocean Coast. Manag.
(2019) Shoreline changes interpreted from multi-temporal aerial photographs and high resolution satellite images: wotje Atoll, Marshall Islands
Remote Sens. Environ.
(2013)- et al.
Formation and adjustment of typhoon-impacted reef islands interpreted from remote imagery: nadikdik Atoll, Marshall Islands
Geomorphology.
(2014) - et al.
Multi-decadal shoreline changes in response to sea level rise in the Marshall Islands
Anthropocene.
(2015) - et al.
Automatic extraction of shorelines from Landsat TM and ETM+ multi-temporal images with subpixel precision
Remote Sens. Environ.
(2012)
A half-century of coastline change in Diego Garcia – the largest atoll island in the Chagos
Geomorphology.
Building beyond land: an overview of coastal land reclamation in 16 global megacities
Appl. Geogr.
Drivers, trends, and potential impacts of long-term coastal reclamation in China from
Estuar. Coast. Shelf Sci.
The dynamic response of reef islands to sea-level rise: evidence from multi-decadal analysis of island change in the Central Pacific
Glob. Planet. Change
Evaluation of various satellite sensors for waterline extraction in a coral reef environment: majuro Atoll, Marshall Islands
Geomorphology.
Object-based cloud and cloud shadow detection in Landsat imagery
Remote Sens. Environ.
Interactions between sea-level rise and wave exposure on reef island dynamics in the Solomon Islands
Environ. Res. Lett.
Urban mapping in landsat images based on normalized difference spectral vector
IEEE Geosci. Remote. Sens. Lett.
Coral reef atoll assessment in the South China Sea using Planet Dove satellites
Remote Sens. Ecol. Conserv.
Projected changes in tropical cyclone activity under future warming scenarios using a high-resolution climate model
Clim. Change
Island building in the South China Sea: detection of turbidity plumes and artificial islands using Landsat and MODIS data
Sci. Rep.
Adapting to climate change in small island developing states
Clim. Change
Historical area and shoreline change of reef islands around Tarawa Atoll
Kiribati. Sustainability Science.
Leveraging public adaptation finance through urban land reclamation: cases from Germany, the Netherlands and the Maldives
Clim. Change
Land raising as a solution to sea‐level rise: an analysis of coastal flooding on an artificial island in the Maldives
J. Flood Risk Manag.
Sea level rise and the ongoing Battle of Tarawa
Eos Trans. Am. Geophys. Union
A global assessment of atoll island planform changes over the past decades
Wiley Interdiscip. Rev. Clim. Change
Rapid human-driven undermining of atoll island capacity to adjust to ocean climate-related pressures
Sci. Rep.
Cited by (19)
Risk classification of low-lying coral reef islands and their exposure to climate threats
2024, Science of the Total EnvironmentI saw the “atoll sign” – and now I need a tropical vacation
2023, Clinical ImagingMulti-decadal planform changes on coral reef islands from atolls and mid-ocean reef platforms of the equatorial Pacific Ocean: Gilbert Islands, Republic of Kiribati
2021, GeomorphologyCitation Excerpt :Within the past decade, a growing number of studies have utilised remotely-sensed datasets to document shoreline changes in coral reef islands across the Pacific (e.g. Webb and Kench, 2010; Ford and Kench, 2015; Duvat and Pillet, 2017; Kench et al., 2018) and the Indian Oceans (Testut et al., 2016; Aslam and Kench, 2017; Duvat and Magnan, 2019). While some studies have reported shoreline erosion and island loss (Albert et al., 2016; Nunn et al., 2017; Garcin et al., 2016), collectively, the majority of islands examined have been stable or have expanded on reef surfaces under relative sea-level rise (McLean and Kench, 2015; Duvat, 2018; Holdaway et al., 2021). Despite widespread concern regarding the future of atoll-nations and a consensus regarding the urgency to understand responses of reef islands to intensifying climate change signals such as accelerating SLR and changes in wave regime, these studies provide a geographically fragmented record of shoreline change.
Multi-decadal atoll-island dynamics in the Indian Ocean Chagos Archipelago
2021, Global and Planetary ChangeCitation Excerpt :Their morphology is influenced by a host of controls, including sea level, on timescales of minutes to millennia. Not least because of our desire to live by the sea, the dynamics and drivers of coastal change have been studied intensively (Richmond, 1993; Kaluwin and Smith, 1997; Dickinson, 1999; Masselink and Pattiaratchi, 2001; Woodroffe, 2008; Collen et al., 2009; Webb and Kench, 2010; Rankey, 2011; Purkis and Klemas, 2011; Taylor and Purkis, 2012; Andréfouët et al., 2013; Purkis et al., 2016a; Duvat and Pillet, 2017; Tuck et al., 2019; Duvat, 2020; Newnham et al., 2020; Holdaway et al., 2021). Notwithstanding raised atolls, those that have been tectonically uplifted, atoll islands are low and flat with maximum elevations typically in the range of only a few meters.
Drivers of shoreline change on Pacific coral reef islands: linking island change to processes
2023, Regional Environmental Change